Act1: Scene IL-25

Interleukin-25 is a member of the IL-17 cytokine family but is responsible for inducing Th2 responses instead of Th17 effects. In fact, IL-25 may negatively regulate the effects of IL-17 and has been implicated in the initiation of allergic airway inflammation like that found in asthma. The “connection to IκB-kinase and SAPK” (CIKS) protein, cloned by Dr. Siebenlist’s laboratory, is also known as the adaptor protein Act1, a name chosen when this molecule was independently cloned by Dr. Xiaoxia Li’s laboratory (see summary below). Claudio et al. (p. 1617
) have demonstrated that the effects of both IL-25 and IL-17 are dependent on the CIKS/Act1 adaptor protein using mice that were developed to be deficient in this molecule. Considering the responses elicited, it was surprising that this adaptor was necessary for the actions of both cytokines. IL-25 exposure induced the production of the Th2 cytokines IL-5, IL-13, and CCL24, as well as mucus hypersecretion and airway hyperreactivity in the lungs of wild-type mice. However, CIKS/Act1-deficient animals did not develop these asthma-like symptoms. The production of these Th2 effects was determined to be through the interaction of IL-25 with CIKS/Act1 through association with the IL-17RB in CD11c+ lung macrophage-like cells. Therefore, the authors have determined that CIKS/Act1 is responsible for the signaling effects of IL-25, using a pathway that it must share in some part with IL-17, a cytokine with appositional effects.

In the companion paper, Swaidani et al. (p. 1631
) performed very similar experiments with their own CIKS/Act1-deficient animals (Act1−/− for clarity) and came to very similar conclusions. The Act1−/− mice from this laboratory share a similar phenotype with the animals from the Siebenlist laboratory; however, they have the additional characteristic of having hyperactive B cell-dependent responses, including splenomegaly, lymphadenopathy, and hypergammaglobulinemia. When stimulated with IL-17 these Act1−/− mice showed reduced airway neutrophil recruitment compared with wild type, and, comparable to the data in Claudio et al., IL-25-induced expression of IL-5, IL-4, IL-13, and eotaxin was ablated in Act−/− mice. Epithelial cell-specific deletion of Act1 caused a loss of IL-17-dependent neutrophilia and IL-25-dependent eosinophilia in response to OVA-induced allergic pulmonary inflammation. The authors of these experiments determined that Act1 expressed within the epithelia plays an essential role in controlling pulmonary inflammation through interaction with IL-17R and IL-25R pathways. Taken together, the two studies provide independently verified support for CIKS/Act1 as a potential target in allergic asthma, pulmonary pathology, and modulation of IL-25/IL-17 pathways.

Epigenetic Development

The accessibility hypothesis suggests that V(D)J recombination is regulated by controlling the access of Ag receptor loci to RAG recombinase-mediated rearrangement. Previous work has supported this hypothesis with the observation that during the development of lymphocytes, germline transcription (GT) of unrearranged genes precedes loci recombination and that gene accessibility is associated with histone posttranslational modifications (PTMs). To probe the accessibility hypothesis, Xu et al. (p. 1362
) undertook a comprehensive ex vivo analysis of PTMs in B and T lymphocytes. The authors looked at four different PTMs of histone H3 using chromatin immunoprecipitation assays. They found that: 1) PTMs were added in a sequential, stage-specific manner at the H and L chain loci, 2) lymphocyte stage-specific PTMs occurring at the Igλ and Igκ loci correlated with the sequential rearrangement; and 3) signaling through the pre-BCR is necessary before PTM occurs at the L chain loci. The use of mice mutant for the κ intronic enhancer also demonstrated that this enhancer is vital for inducing GT as well as PTMs. Experiments with an inducible pre-B cell line indicated that PTMs at the Jκ loci occur after GT initiation and that GT drives PTMs rather than PTMs making genes accessible for transcription. Taken together, the data provide an epigenetic profile of IgL genes, one that is determined by pre-BCR signals and the developmental stage of the B lymphocyte.

It Only Takes One

Help from T cells is necessary for B cells to produce IgG, which is an integral part of the immune response to both infections and autoimmune diseases. However, whether multiple T cell clones or a single clone is necessary to facilitate an in vivo polyclonal IgG response remains unclear. Using a mouse model of the fatal autoimmune skin disease pemphigus vulgaris, Takahashi et al. (p. 1740
) examined the production of IgG autoantibodies against desmoglein 3 (Dsg3). Using previously isolated Dsg3-reactive T cell clones from Dsg3−/− mice, the authors adoptively transferred a single T cell clone with unprimed B cells from Dsg3−/− mice. Recipient mice produced IgG Abs that recognized at least three parts of the Dsg3 extracellular domain. Individual recipient mice demonstrated at least two subclasses of anti-Dsg3 IgGs (i.e., IgG1, IgG2a, IgG2b, and IgG3). Pathogenic Dsg3-reactive T cell clones recognized different epitopes but stimulated the production of anti-Dsg3 Abs that had a similar epitope recognition profile. These Abs tended to recognize epitopes from the middle to the C-terminal extracellular domain of Dsg3, which differs from Abs found in pemphigus vulgaris patients. The authors have demonstrated that polyclonal anti-Dsg3 Abs with a similar epitope-binding distribution can be generated from a single T cell clone and unprimed B cells.

How T Cells Home

The expression of chemokines, including CCL21, CCL19, CXCL12, and CXCL13, on the lumen and basal lamina of high endothelial venules (HEVs) in lymph nodes (LNs) and Peyer’s patches (PPs) indicates a pathway for lymphocyte migration into these lymphoid organs. Bai et al. (p. 1287
) determined that T cells migrated across HEVs through the action of CXCL12 and its receptor CXCR4 in cooperation with CCR7 ligands. In vitro, under suboptimal CCR7-ligand concentrations, T cell migration from wild-type but not CXCR4−/− mice was enhanced by CXCL12. This enhancement was not due to an increase in the expression level or ligand-binding ability of CCR7. CXCL12 also enhanced ERK phosphorylation levels and actin polymerization in T cells that were initiated by CCR7 ligand stimulation. When naive T cells from wild-type mice were pretreated with CXCL12 and adoptively transferred into wild-type or CCR7 ligand−/− recipient mice, T cell migration to the LNs and PPs was increased in the wild-type but not in CCR7 ligand−/− mice. The mechanism of migration was dependent on enhanced T cell binding to the HEVs and trafficking to the LN parenchyma. Thus, the authors demonstrate that CXCL12 enhances CCR7-ligand-dependent T cell migration across the HEVs and provides an efficient mechanism for entry into PPs and LNs.

The Chicken and the Ig

The chicken Ig-like receptors (CHIR) comprise a large immunoregulatory Ig-like receptor family, are normally encoded by the chicken leukocyte receptor complex (LRC) on chromosome 30, and have shared homology with mammalian leukocyte Ig-like receptors (LILR) and killer cell Ig-like receptors (KIR). However, the CHIR demonstrate greater variety than their mammalian LILR and KIR counterparts, displaying greater haplotypic and allelic variation. To build on previous work that identified CHIR-AB1 as a high affinity Fc receptor, Viertlboeck et al. (p. 1533
) used database search methods to identify another receptor with chicken Ig-binding ability. Through the database search, they were able to clone a novel Fc receptor they named Gallus gallus FcR (ggFcR). ggFcR selectively bound IgY and consisted of four extracellular constant 2-set (C2-set) Ig domains, an arginine-containing transmembrane domain, and a short cytoplasmic tail. As is indicative of an activation receptor, ggFcR associated with the common γ-chain. RNA expression analysis determined that this novel receptor was highly expressed in PBMC, spleen cells, thrombocytes, and macrophages. Similar to most Ig-like receptor genes, the signal peptide of ggFcR was encoded by two separate exons, one of which had the signature 36 bp common to genes encoded in the LRC. However, in a twist that indicated the further complexity of the CHIR, the ggFcR was not encoded in the LRC but as an isolated gene on chromosome 20. Thus, the authors have identified a novel, potentially activating IgY FcR with expression ranging from PBMCs to thrombocytes in a unique chromosomal location.

Educating Natural Treg

Natural regulatory T cells (nTreg) are generated in the thymus and suppress the action of autoreactive lymphocytes. Foxp3 is known to be important for Treg function in tolerance. However, what role this transcription factor plays in thymic signals and nTreg development is unclear. Relland et al. (p. 1341
) have used altered peptide ligands with established affinities in the N3.L2 TCR transgenic mouse model to determine how nTreg recognition of self-peptides controls selection. The authors determined that like conventional T cells (Tconv), the nTreg TCR repertoire is selected on the basis of TCR-ligand affinity. In neonatal thymic organ culture, peptides with agonist activity generated nTreg and led to negative selection of Tconv. Foxp3 expression was not dependent on a high-affinity TCR-MHC interaction for induction, but did require the binding of a strong peptide agonist. Hence, this indicates that peptide agonist binding stimulated Foxp3 expression in an already precommitted Treg. Kinetic analysis determined that Treg are controlled through a process of positive and negative selection that is separate from agonist-mediated Foxp3 expression. TGF-β signaling enhanced the production of Treg from thymic precursors. Thus, the authors determined that the timing of ligand exposure and affinity, TGF-β signaling, and thymic microenvironment are all important in the development of this enigmatic cell type.

Achieving Cross-Presentation

Antitumor immunity is better achieved through vaccines that cause cross-presentation of a vaccination Ag. Previous work from this group demonstrated that vaccine formulations of full-length cancer/testis Ag NY-ESO-1 as an immune complex (NY-ESO-1/IC) or with ISCOMATRIX (CSL Ltd.) (NY-ESO-1/ISCOMATRIX) could be efficiently cross-presented by dendritic cells to CTL. Schnurr et al. (p. 1253
) have now determined that NY-ESO-1/ICs can present HLA-A2 and HLA-Cw3 epitopes through a proteasome-dependent pathway. The NY-ESO-1/ISCOMATRIX achieved cross-presentation through the use of the cytosolic protease tripeptidyl peptidase II (TPPII). This was unlike the NY-ESO-1/IC vaccine, which used a proteasome-dependent pathway. Dendritic cells used endocytosis to take up both vaccine formulations, and trafficking studies showed delivery to lysosomes. In contrast to the Ag from NY-ESO-1/IC, which was retained in the cytosol, Ag from the NY-ESO-1/ISCOMATRIX vaccine quickly translocated to the cytosol. Thus, it could interact with the TPPII protease. This Ag translocation was unique to dendritic cells differentiated from IL-4-treated monocytes. Taken together, the data point to an interesting mechanism of Ag processing that supports cross-presentation of CTL epitopes, uses cytosolic TPPII, and could be important for formulation of vaccines.